Anytime a transmission shows signs of a problem, such as harsh shifting or slipping, the repairs can be a minor fix. Other times the transmission will need a complete rebuild, if the problem is caused by a major defect. Whether you should replace the front pump at the same time as you perform a rebuild is determined by the condition of the connecting components.
The automatic transmission has two pumps that work with the transmission. One of those pumps is the front pump that pushes transmission fluid through the valve body. The front pump is responsible for providing all the oil pressure the transmission requires. The front shaft of the transmission works in conjunction with the front pump. If you replace the reaction shaft, you must replace the front pump. If the reaction shaft can be cleaned and reused, it is best to rebuild the old front pump and reinstall the component.